Casting process



United States Patent M 3,022,556 CASI'ING PROCESS George R. Gardner, Berea, and John C. Buck, Solon, Ohio, assignors to Aluminum Company of America, Pittsburgh, Pa, a corporation of Pennsylvania No Drawing. Filed Jan. 30, 1959, Ser. No. 790,044 6 China. (Cl. 22-193) This invention relates to a method of casting, and more particularly to a method of casting light metal, thin walled articles. As used herein, the term light metal includes aluminum and magnesium and the alloys in which these elements constitute at least 50% by weight of the total composition. As used herein, the term thin walled includes castings which have a wall thickness on the order of from about ,4, to V8 of an inch.

Modern aircraft contain a maze of light metal ducts which control the flow of air for heating, ventilating and the like. It is difiicult to make light metal castings having thin walls less than M; of an inch in thickness, and therefore these ducts presently are fabricated from sheet by die forming and welding and riveting. One of the principal reasons for the difliculty in making such thin walled castings is the fact that the molten metal freezes before it reaches the remote portions of the mold cavity. Also, tolerances on wall thickness are extremely important, and even a slight misalignment of mold components may cause a warm.

In the well-known investment casting or lost wax process, it is customary to form an expendable wax pattern which is subsequently invested with a suitable plaster or ceramic mold material. The mold is then heated to a very high temperature to melt the wax pattern and to burn or completely carbonize any residual wax remaining in the mold cavity.

It has been found that the general principle of lost wax founding, that is, the replacement of a wax pattern with cast metal, can be employed in the production of large, thin walled castings by employing several materials and procedures hereinafter described. The process is especially applicable to the production of intricately shaped air ducts for aircraft. Heretofore, the lost wax process has not been considered adaptable for making such parts because of their large size. Also, such parts are considered very costly to make as sand or plaster castings because of the extreme accuracy required of the pattern and core box equipment in order to produce mold sections accurate enough to maintain the thin wall dimen- SIOHS.

It is therefore the primary object of this invention to provide a method of casting light metal articles, a substantial portion of which consists of flat or curved surfaces and thin walls of accurately controlled thickness. It is a further object to apply an expendable wax pattern to a permeable plaster mold for producing a thin walled casting. It is still a further object to produce a low cost casting in a rapid manner without sacrifice in surface finish of the casting produced nor in the accuracy of the thickness of the thin wall.

In accordance with the present invention, a light metal casting can be made having a relatively thin walled portion of less than 96 inch in thickness by employing a permeable gypsum plaster mold having a mold cavity formed from a thin wax pattern. When the mold-wax assembly is heated to a moderately low temperature and above the melting point of the wax, the wax melts and is either absorbed by the porous plaster or discharged 'from the cavity through suitably placed openings thereby leaving a clean thin and accurate mold cavity. Care must be exercised to avoid excessive temperatures and carbonization of the wax. The molten light metal may be poured into the cavity under normal atmospheric pressure to form 3,022,556 Patented Feb. 27, 1962 the thin walled metal casting. Better results are often secured if the mold is partially evacuated before the metal is introduced. By this method, it is possible to make both non-cored and cup-shaped castings as well as hollow castings having substantial thin walled portions.

The permeable gypsum plaster mold used may be of the type described in US. Patent No. 2,754,220 to George R. Gardner. The permeability is created by forming a foam of the slurry and allowing the foam to set. The permeability which is satisfactory for our purposes should be no less than 20 as measured on the American Foundrymen Society (A.F.S.) scale, and usually is on the order of 20 to 200. Because of its low chilling capacity, permeable plaster is well suited to the production of thin walled castings made in accordance with our invention. Additions of long fibered talc, other mineral fibers, sand and the like to the permeable plaster mixes for cores and molds help in preventing excessive loss of strength in these components during the heating periods required to dry the plaster and melt the wax and when the mold is filled with the molten metal. Permeability of the plaster mold is not significantly affected by the wax absorption. However, care must be taken not to carbonize the wax absorbed by the porous plaster because this will render the mold useless.

In making a permeable plaster mold for the production of thin walled, hollow castings, for example, a permeable plaster core which defines the internal cavity of the casting is prepared. For example, in making a duct, a wood core pattern is made corresponding to the internal cavity of the casting including the required core prints. From' this pattern, a rubber lined core box is made by standard procedures. The plaster slurry is poured into the core box to form the desired plaster core for the duct mold. The wax may be applied either in the form of sheet or by dipping the core in molten wax bath, as described below. If sheet wax is used, a sheet of this material is employed having the same thickness as that desired for the walls of the casting and pieces are cut therefrom to fit the plaster core. The cut pieces of wax may be fastened to the core by fusing them to each other at the edges or corners if the core has an angular contour and thus form a tightly fitting sleeve or mantle. In the case of large or curved core surfaces, or with cylindrically shaped cores, the wax sheets may be securely fastened to the core by means of a suitable adhesive. This may be applied to the entire surface of the wax sheet which in turn is applied to the core. The pre-cut sheets of wax may be applied to either a wet or dry plaster core with equally good results. The pattern wax employed is of conventional composition for use in lost wax casting methods and must have sufiicient plasticity to permit any necessary bending or shaping, but it should not be too soft or tacky so that it cannot be readily handled at room temperature.

Although it is generally more convenient and efiicient to apply the wax pattern to the core, it is to be understood that the opposite procedure can be followed, namely, to first form the external mold portion, apply the wax pattern thereto and invest it with a core. in making non-cored castings having thin walled portions, the wax pattern can be applied to any mold section and another plaster mold section formed in contact with the pattern.

Flanges and bosses which may be required on the duct can also be formed by cutting or molding wax to the proper shape and fastening them to the main wax assembly. When the entire wax sheathing has been formed around the permeable plaster core, wax patterns of sprues, gates and risers may be attached to the sheath as required, and the completed wax assembly invested with permeable plaster to form the external portion of the mold. It is obvious that thin walled, non-cored castings, particularly those having an irregular or rib shape,

can be made in substantially the same manner.

An alternative method for applying the wax pattern to the permeable plaster core or other mold part is particularly applicable to more complex cores or parts having surfaces with compound curvatures. The wax pattern is applied to the permeable plaster mold by immersing the core or part in molten wax preferably floating on a water bath. A thin, continuous coating of wax is deposited on the core and can be increased to any desired thickness by repeated immersions.

According to this method, a layer of molten wax is floated on the surface of a tank of relatively cool water, the main body of which is maintained at a temperature substantially below that of the melting point of the wax. The core may be initially immersed or sprayed or otherwise wetted with cool water to displace part of the air at the surface thereby preventing the wax from penetrating the pores of the core, and also to increase the rate of coating build-up by promoting chilling at the core surface. As the core is lowered through the molten wax layer, a thin, continuous film of wax solidifies against the cool core. Further chilling of the wax takes place after the core has passed below the wax layer and into the cooler water. After a short holding period to allow adequate cooling of the deposited wax film, the core is withdrawn through the layer of wax thereby acquiring a second coating. The core must be raised above the molten wax in order to permit the wax coating to solidify. This process may be repeated until a wax layer of the desired thickness has been built up upon the core P After the coating has cooled to room temperature any required flanges, made from pre-cut wax, may be fastened to the core, and the pre-cut wax pattern sealed in place as described above. Similarly, the required wax pattern gating system is next applied. The core assembly is suitably supported and a plaster slurry poured around it.

In producing a cup-shaped casting by employing a hollow core of that shape, it may be desirable to cover the opening in the core with a lid, or otherwise plug the opening. Thus when the core-wax assembly is immersed in the molten wax, no wax is deposited on that portion of the core that is covered.

- Use of a relatively thin layer of wax floated on a water bath has an advantage over the use of a full tank of molten wax, because each part of the core is exposed to the wax for the same period of time and acquires a coating of uniform thickness. When a full tank of wax is used, the bottom of the core is exposed to the wax for a considerably longer period than the top, and a tapered coating will result. This is undesirable where uniformity of wall thickness is of considerable importance as an aircraft duct component. The depth of the molten wax layer is critical only insofar that there must be sufiicient wax in excess of that required to properly I coat the plaster core because a distinct layer of wax, and

not a wax-water mixture, must be maintained during the immersing operation to avoid the capture of water bubbles in the coating. However, it is not necessary, nor desirable, that the molten wax be of suflicient depth to completely surround the entire core when it is submerged.

Accurate control of immersion tank temperature, lifting and lowering rates and free water content of plaster core is essential to successful wax immersion coating. It should be understood that these variables are somewhat dependent upon the geometry of the core, and may be readily determined by one skilled in the art.

The desired temperature of the molten wax may be maintained by an induction or resistance heater sur rounding the container at about the level of the wax layer. The temperature necessary to maintain the wax molten should not be high enough to cause the formation of steam below the wax layer. The wax employed should have a melting point between about to 212 F., and more preferably between about to 200 F., but should be sufliciently high so that a thin coating formed on the core will harden in a relatively short time when submerged in the cool water bath or when exposed to the atmosphere. In addition, the wax should be sufiiciently stable so as to withstand prolonged use after exposure to the water in the immersion tank.

The main body of the water bath is maintained preferably below about 110 F., but may be somewhat higher depending upon the melting point of the wax employed. A cool water bath is desirable in that it prevents remelting of portions of wax previously deposited. Any excessive temperature rise in the water bath is combated by circulating cold water through a coil submerged in the water bath a few inches beneath the heated wax layer. The portion of the water bath adjacent to the molten wax approaches the temperatur of the wax, but in this manner a high thermal gradient between the wax layer and main body of the water bath is maintained. The free water content of the core is fairly critical and depends upon the core shape and size. Too little water allows rapid heating and expansion of the air in the core and leads to bubbles and blisters'in the wax coating. This is particularly true with extended immersions in hot water. An excess of free water may cause water blisters to form near the bottom of the core after withdrawal from the tank and occasional splitting of the wax coating. as a result of too rapid chilling.

The mold assembly with the wax pattern in place, made in accordance with our invention, is heated in a suitable oven, such as a gas fired oven, electrically heated oven, or a dielectric drying oven, to evaporate at least a portion of the free water content of the plaster and to melt the wax pattern, but excessive temperatures must be avoided in order to prevent carbonization of wax. The molten wax disappears from the mold cavity by one of two mechanisms, or both. The molten wax may be ab- 7 sorbcd by the plaster because of its porous nature. Where a substantial portion of the melted wax is not absotbed by the plaster, the wax may be drained out through one or more suitably placed openings. The latter situation may occur when the heating rate is sufficiently rapid to melt the wax without substantial evaporation of the water from the plaster. In either case, removal of the wax leaves a mold containing a core which is incapable of shifting and a clean cavity having the shape of the desired casting and required gating system.

The molten light metal is poured within the usualcasting temperature range of about 1250 to 1500' F., and solidifies at a fast enough rate to produce a satisfactory casting. Better results are often obtained if the entire plaster mold is placed within a chamber which is partially evacuated before the metal is poured, only the pouring box of the mold extending outside of the chamber. The reduced pressure within the chamber causes the mold gases to escape from the mold cavity at a sufficiently rapid rate to prevent misruns and cold shuts. However, it may not be necessary to partially evacuate the mold when suflicient ventage is provided. The surface of the casting has the smooth characteristics imparted by the plaster surface plus the precision imparted by the lost wax pattern.

The following examples will further illustrate the practice of the invention.

A rectangular casting having dimensions 2 x 4 x 8 inches and a wall thickness of M; of an inch was made in accordance with our invention. Precut sheets of wax 54 inch thick were placed over the permeable plaster core made by conventional procedures and held in place by fusing the wax at the edges with a heated tool. The core prints extended approximately 2 inches beyond the wax at each end. A 35 inch thick web gate pattern was applied to one of the 8 inch edges, and a sprue pattern of wax was in turn attached to the opposite edge of the web gate. The completed assembly was supported in a wood frame, and a foamed plaster slurry poured around it. The mold was dried in a dielectric oven for 20 minutes, and a portion of the melted wax was absorbed by the porous plaster, and the remainder being drained out through the sprue cavity. The mold was placed in a vacuum chamber with the molds pouring box projecting therefrom, and a vacuum of 6 inches of water was applied during pouring. An aluminum alloy having 7% silicon and 0.3% magnesium was poured into the mold cavity at a temperature of 1350' F. to produce the thin wall duct casting. A smooth surfaced casting was produced having a uniform wall thickness ,6 inch in thickness.

A duct part 35 inches long of 3 inch diameter circular section at one end, a 4% x 1% inch oval section in the center portion and a 3 x 6 inch oval section at the opposite end was cast around a core which had been coated with wax by the immersion method. A flange approximately 1 inch wide was incorporated around the oval end of the duct.

The pattern equipment required consisted of two core boxes, one for a core to form the internal surface of the duct and the other to produce a core to support the wax flange pattern which was cut from sheet wax and attached after the immersion coating was complete. Plaster backed, rubber lined core boxes were constructed by conventional procedures for this purpose.

An immersion tank 12 inches in diameter and 48 inches high was filled to a depth of 40 inches with water. A three inch layer of molten wax was floated on the water and maintained at a temperature of from 188 F. to 196 F. by an immersion heater. A cooling coil immersed in the water maintained a water temperature of about 106 F. The core was first immersed in a cold water bath and while still wet passed through the molten wax layer and into the underlying water bath. The core was lowered and lifted at a rate of 2% feet per minute, was held for 20 seconds in the cold water bath beneath the wax layer and for 10 minutes in air before the next immersion. By repeated immersions, a inch wax layer was achieved, and the core was allowed to cool to room temperature. The wax pattern for the end flange was attached at one end of the wax sheath by an adhesive. The core assembly was supported in a wood frame, and a foamed plaster slurry poured around it.

The mold and wax coated core were dried in a conventional oven for 48 hours at 250' F. whereby the wax was melted and absorbed by the plaster. The mold assembly was placed in a chamber with the pouring sprue extending to the exterior, and the pressure within the chamber reduced by 20 to 30 inches of water. The aluminum described in the prior example was poured at 1400 F. thereby forming the thin wall duct having uniform wall dimensions.

Having thus described our invention and embodiments thereof, we claim:

1. A method of making light metal, thin walled castings comprising forming a part of permeable gypsum plaster, immersing the part in a bath of molten pattern was: and withdrawing it therefrom thereby depositing a thin, continuous coating of wax on that portion of said part which is to come in contact with the molten metal, repeating said immersion in and withdrawal from said wax bath, if necessary, to build up a wax layer of the desired thickness, investing the wax and mold part assembly with a permeable gypsum plaster to form the external portion of the mold, said plaster mold parts being formed from a plaster slurry capable of yielding a plaster having a minimum permeability of 20 on the A.F.S scale, heating said mold and wax assembly to a high enough temperature to melt the wax without carbonization, draining ofi any excess of molten wax from the mold not absorbed by the plaster thereby providing the desired mold cavity, and thereafter introducing the molten light metal into said mold cavity to form the thin walled casting.

2. The method according to claim 1, wherein said permeable gypsum plaster part is wetted with cool water before immersion in the molten wax bath.

3. A method of making light metal, thin walled castings comprising forming a core of permeable gypsum plaster, depositing a thin, continuous and uniform film of pattern wax on that portion of said core which is to come in contact with the molten metal by immersing said core in a body of molten pattern wax floating on a water bath maintained at a lower temperature, withdrawing said core and repeating said immersion and withdrawal until the desired thickness of wax coating is developed, investing the core-wax assembly with a permeable gypsum plaster to form the external portion of the mold, said plaster mold and core parts being formed from a plaster slurry capable of yielding a plaster having a minimum permeability of 20 on the A.F.S. scale, heating said mold and core-wax assembly to a high enough temperature to melt the wax without carbonization, draining off any excess of the molten wax from the mold not absorbed by the plaster thereby providing the desired mold cavity, and thereafter introducing the molten light metal into the mold cavity to form the thin walled casting.

4. A method of making light metal, thin walled, hollow castings comprising forming a core of permeable gypsum plaster, depositing a thin, continuous and uniform film of pattern wax on that portion of said core which is to come in contact with the molten metal by immersing said core in a body of molten pattern wax floating on a water bath maintained at a lower temperature, withdrawing said core and repeating said immersion and withdrawal until the desired thickness of wax coating is develcped, investing the core-wax assembly with a permeable gypsum plaster to form the external portion of the mold, said plaster mold and core parts being formed from a plaster slurry capable of yielding a plaster having a minimum permeability of 20 on the A.F.S. scale, heating said mold and core-wax assembly to a high enough temperature to melt the wax without carbonization, draining off any excess of the molten wax from the mold not absorbed by the plaster thereby providing the desired mold cavity, partially evacuating the mold cavity, and thereafter introducing the molten light metal into the mold cavity to form the thin walled casting.

5. The method according to claim 4 wherein the melting point of the wax is between about 130 and 212 F.

6. The method according to claim 4 wherein the temperature of the water is below about F.

References Cited in the file of this patent UNITED STATES PATENTS 1,325,004 Davidson Dec. 16, 1919 2,205,006 Eccles June 18, 1940 2,362,875 Zahn Nov. 14, 1944 2,756,475 Hanink July 31, 1956 OTHER REFERENCES Metal Industry publication (Precision Casting), Dec. 19, 1949, page 497 relied on.

Investment Castings for Engineers, by R. L. Wood and D. Von Ludwig (1952), page 141 relied on. 

1. A METHOD OF MAKING LIGHT METAL, THIN WALLED CASTINGS COMPRISING FORMING A PART OF PERMEABLE GYPSUM PLASTER, IMMERSING THE PART IN A BATH OF MOLTEN PATTERN WAX AND WITHDRAWING IT THEREFROM THEREBY DEPOSITING A THIN, CONTINUOUS COATING OF WAX ON THAT PORTION OF SAID PART WHICH IS TO COME IN CONTACT WITH THE MOLTEN METAL, REPEATING SAID IMMERSION IN AND WITHDRAWAL FROM SAID WAX BATH, IF NECESSARY, TO BUILD UP A WAX LAYER OF THE DESIRED THICKNESS, INVESTING THE WAX AND MOLD PART ASSEMBLY WITH A PERMEABLE GYPSUM PLASTER TO FORM THE EXTERNAL PORTION OF THE MOLD, SAID PLASTER TO FORM THE EXTERNAL FROM A PLASTER SLURRY CAPABLE OF YIELDING A PLASTER HAVING A MINMUM PERMEABILITY OF 20 ON THE A.F.S SCALE, HEATING SAID MOLD AND WAX ASSEMBLY TO A HIGH ENOUGH TEMPERATURE TO MELT THE WAX WITHOUT CARBONIZATION, DRAINING OFF ANY EXCESS OF MOLTEN WAX FROM THE MOLD NOT ABSORBED BY THE PLASTER THERBY PROVIDING THE DESIRED MOLD CAVITY, AND THEREAFTER INTRODUCING THE MOLTEN LIGHT METAL INTO SAID MOLD CAVITY TO FORM THE THIN WALLED CASTING. 